feat(05-04): add query_model_direct() for @mention support

Add direct model query function to orchestrator that:
- Takes single model, message, optional discussion context, and project name
- Builds context from discussion history when available
- Uses system prompt indicating this is a direct message
- Handles errors gracefully (returns error message string)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/moai/core/orchestrator.py

@@ -107,6 +107,58 @@ def build_context(discussion: Discussion) -> list[dict]:
     return messages
 
 
+async def query_model_direct(
+    model: str,
+    message: str,
+    discussion: Discussion | None,
+    project_name: str,
+) -> str:
+    """Query a single model directly with optional discussion context.
+
+    Used for @mention messages where user addresses a specific model.
+    If a discussion is provided, includes full context so the model
+    can reference prior responses.
+
+    Args:
+        model: Model short name (e.g., "claude", "gpt", "gemini").
+        message: The direct message to the model.
+        discussion: Optional Discussion object for context (with eager-loaded rounds/messages).
+        project_name: Project name for context.
+
+    Returns:
+        The model's response text, or error message if the query fails.
+    """
+    client = get_ai_client()
+
+    # Build system prompt indicating this is a direct message
+    system_prompt = f"""You are participating in a discussion about: {project_name}
+
+This is a direct message to you specifically. The user has chosen to address you
+directly for your unique perspective.
+
+Respond helpfully and concisely."""
+
+    # Build messages with optional discussion context
+    if discussion is not None:
+        messages = build_context(discussion)
+        # Add the direct message
+        messages.append({"role": "user", "content": f"[Direct to you]: {message}"})
+    else:
+        messages = [{"role": "user", "content": message}]
+
+    try:
+        response = await client.complete(
+            model=model,
+            messages=messages,
+            system_prompt=system_prompt,
+        )
+        logger.info("Direct query to %s successful", model)
+        return response
+    except Exception as e:
+        logger.error("Direct query to %s failed: %s", model, e)
+        return f"[Error: {e}]"
+
+
 async def run_discussion_round(
     discussion: Discussion,
     models: list[str],